[Thumbnails DB] Add functionality to clear unused on-demand favicons.

components/history/core/browser/expire_history_backend.h

Index: components/history/core/browser/expire_history_backend.h
diff --git a/components/history/core/browser/expire_history_backend.h b/components/history/core/browser/expire_history_backend.h
index ebffc5b62e4e14ee9b45b271c18c0a4f1333b326..8fb3fe24ee26e89897426ae2723a49d998d185e6 100644
--- a/components/history/core/browser/expire_history_backend.h
+++ b/components/history/core/browser/expire_history_backend.h
@@ -179,6 +179,9 @@ class ExpireHistoryBackend {
   // any now-unused favicons.
   void ExpireURLsForVisits(const VisitVector& visits, DeleteEffects* effects);
 
+  // Clears all old on-demand favicons from thumbnail database.
+  void ClearOldOnDemandFavicons();
+
   // Deletes the favicons listed in |effects->affected_favicons| if they are
   // unsued. Fails silently (we don't care about favicons so much, so don't want
   // to stop everything if it fails). Fills |expired_favicons| with the set of